In the United States alone, about 44 million people are estimated to have osteoporosis or at risk of developing osteoporosis due to decreased bone mass and density. Annually, osteoporosis is responsible for millions of bone fractures that severely affect the quality of life. This is particularly significant in women soon after menopause due to estrogen deficiency, a condition referred to as postmenopausal osteoporosis. Estrogen deficiency is associated with increased osteoclast (OC) activation, decreased osteoblast (OB) function and increased inflammatory bone-resorbing cytokines such as interleukin-6 and tumor necrosis factor (TNF?). Several targeted therapies are currently available to treat and/or prevent osteoporosis by blocking OC activity. However, evidence has shown that long-term treatments have caused a reduction in bone formation by OBs, resulting in atypical skeletal fractures. In this proposal, we put forward the notion that an ideal therapeutic scenario would be one that impairs OC function without interfering with OB-driven bone formation. Sealing ring formation is a requirement for normal OC function. We recently identified the actin bundling protein L-Plastin (LPL) as a critical factor in the assembly of precursor or nascent sealing zones (NSZs) at the early phase of sealing ring formation. Our preliminary findings show that the TNF? signaling pathway regulates this assembly by mediating LPL phosphorylation. In addition, our data strongly suggest that LPL plays a major role in bone remodeling since LPL-/- mice are osteopetrotic. OC bone-resorbing capacity in these mice is significantly impaired, while OB function remains unaltered. Despite progress in the field, many gaps in knowledge are still unsolved relative to the biology of sealing ring formation in OCs. In particular, little is known about NSZs and the role of LPL in OCs. The proposed studies will explore the essential function of LPL phosphorylation in OC function and bone loss. Our overall goal is to identify LPL as a potential therapeutic target for OC-mediated bone loss. This proposal will test the central hypotheses that LPL is a key regulator of OC bone resorptive function. Inhibiting LPL phosphorylation will attenuate osteoporosis-associated bone loss. We propose the following three specific aims: 1) To determine the role of L-plastin in NSZ formation, independently of integrin ?v?3 signaling in osteoclasts; 2) To elucidate the essential function of L-plastin phosphorylation by TNF? in actin bundling, a process required for NSZ formation in osteoclasts, and 3) To determine the impact of inhibiting endogenous L-plastin phosphorylation in aging- and ovariectomy- induced bone loss in vivo. The outcome of the proposed studies will elucidate the ability of LPL inhibitory peptides to impair OC function and reduce bone loss in mouse models in vivo without affecting OB function. Osteoporosis is related to estrogen deficiency and aging. It remains a significant public health problem and current treatment options have important limitations. Therefore, novel and improved therapies are critically needed to more efficiently target osteoporosis. We anticipate that the outcomes of these studies will provide a translationally relevant foundation on which novel prevention and treatment options for osteoporosis can be achieved. Our results may ultimately impact treatment of other bone loss-associated diseases, including rheumatoid arthritis and periodontitis, which share several pathologic features with osteoporosis.